1. Field of the Invention
This invention relates to a process for dewaxing hydrocarbon oils by a catalytic hydroisomerization process.
2. Background of the Art
Processes for dewaxing petroleum distillates have been known for a long time. Dewaxing is required when highly paraffinic oils are to be used in products which need to remain mobile at low temperatures e.g. lubricating oils, heating oils, jet fuels. The higher molecular weight straight chain normal and slightly branched paraffins which are present in oils of this kind are waxes which are the cause of high pour points in the oils and if adequately low pour points are to be obtained, these waxes must be wholly or partly removed. In the past, various solvent removal techniques were used e.g. propane dewaxing, MEK dewaxing, but the decrease in demand for petroleum waxes as such, together with the increased demand for gasoline and distillate fuels, has made it desirable to find processes which not only remove the waxy components but which also convert these components into other materials of higher value. Catalytic dewaxing processes achieve this end by selectively cracking the longer chain n-paraffins, to produce lower molecular weight products which may be removed by distillation. Processes of this kind are described, for example, in U.S. Pat. No. 3,668,113.
Zeolites, both natural and synthetic, crystalline materials (usually aluminosilicates), can be employed as catalysts for dewaxing, as well as for other processes. The feature which gives zeolite their special utility as catalysts is that they are porous microcrystalline structures. Pore size and shape determine the selectivity of a particular type of zeolite for particular reaction products. Catalytic behavior of a zeolite is also affected by the composition, i.e., its silica/alumina ratio. Zeolite are often used in conjunction with other materials, such as metals, e.g., platinum, nickel, molybdenum, tungsten and/or oxides, e.g., Al.sub.2 O.sub.3, SiO.sub.2.
The crystal structure of a zeolite may be determined by x-ray diffraction analysis, which gives a diffraction pattern unique to each type of zeolite.
In order to obtain the desired selectivity for dewaxing, the catalyst has usually been a zeolite having a pore size which admits the straight chain n-paraffins either alone or with only slightly branched chain paraffins, but which excludes more highly branched materials, cycloaliphatics and aromatics. Zeolites such as ZSM-5, ZSM-11, ZSM-12, ZSM-23, ZSM-35 and ZSM-38 have been proposed for this purpose in dewaxing processes and their use is described in U.S. Pat. Nos. 3,894,938; 4,176,050; 4,181,598; 4,222,855; 4,229,282 and 4,247,388. A dewaxing process employing synthetic offretite is described in U.S. Pat. No. 4,259,174. A hydrocracking process employing zeolite beta as the acidic component is described in U.S. Pat. No. 3,923,641.
Since dewaxing processes of this kind function by means of cracking reactions, a number of useful products become degraded to lower molecular weight materials. For example, olefins and naphthalenes may be cracked down to butane, propane, ethane and methane and so may the lighter n-paraffins which do not, in any event, contribute to the waxy nature of the oil. Because these lighter products are generally of lower value than the higher molecular weight materials, it would obviously be desirable to avoid or to limit the degree of cracking which takes place during a catalytic dewaxing process, but to this problem there has as yet been no solution.
Another unit process frequently encountered in petroleum refining is isomerization. In this process, as conventionally operated, low molecular weight C.sub.4 to C.sub.6 n-paraffins are converted to iso-paraffins in the presence of an acidic catalyst such as aluminum chloride or an acidic zeolite as described in G.B. No. 1,210,335. Isomerization processes for pentane and hexane which operate in the presence of hydrogen have also been proposed but since these processes operate at relatively high temperatures and pressures, the isomerization is accompanied by extensive cracking induced by the acidic catalyst, so that, once more, a substantial proportion of useful products is degraded to less valuable lighter fractions.
However, distillate feedstocks can be effectively dewaxed by isomerizing the waxy paraffins without substantial cracking if the isomerization is carried out over zeolite beta as a catalyst, especially if the catalyst includes a hydrogenation/dehydrogenation component such as platinum or palladium. The hydrogenation/dehydrogenation component may be used in the absence of added hydrogen to promote certain hydrogenation-dehydrogenation reactions which will take place during the isomerization.
The process is carried out at elevated temperature and pressure. Temperatures will normally be from 250.degree. C. to 500.degree. C. (about 480.degree. F. to 930.degree. F.) and pressures from atmospheric up to 25,000 kPa (3,600 psig). Space velocities will normally be from 0.1 to 20. A description of such a process may be found in U.S. Pat. No. 4,419,220, herein incorporated by reference in its entirety.
U.S. Pat. No. 4,601,933 discloses a process for dewaxing lubricating oil feedstocks by passing a waxy feedstock over a catalyst bed containing a mixture of a medium pore zeolite and a large pore zeolite having isomerization activity.
Further information about catalytic dewaxing may be found in U.S. Pat. Nos. 3,894,938; 4,176,050; 4,358,362; and 4,181,598.